# [Proj] Dataset mismatch?

Ed McNierney ed at topozone.com
Fri Jan 27 13:29:26 EST 2006

```Mike -

Yes, the USGS used thousands of points, but they needed to cover the
whole country!  For CONUS that's around 50,000 quads worth.  We're only
talking about one quad that's only about 10 x 15 kilometers.  If the NGS
used 400 points per quad they would have needed over 20 million points.

I took a look at the Mount Sherman, CO quad that is the subject of the
original post.  The northwest corner of that quad is at 392131E 4345059N
(NAD27) and 392084E 4345267N (NAD83).  The NAD27 -> NAD83 datum shift at
that point is (-47E, +208N).  If I go to the southeast corner of that
quad, the coordinates are 402746E 4331044N (NAD27) and 402699E 4331252N
(NAD93).  The NAD27 -> NAD83 datum shift at that opposite corner is
(-47E, +208N) - identical to the datum shift at the opposite corner!  I
cannot imagine one would require 400 intermediate control points to
accurately calculate a datum shift across an area the size of a
7.5-minute quad - especially when the shift at opposite corners is the
same.  You might, I suppose, manage to get off by a meter or two, but
since a single pixel on a 1:24K DRG is 2.4384 meters on a side, you're
literally below the threshold of precision on the data.

Jarrett's seeing discrepancies of hundreds of meters.  Even if you
measured only one point and presumed the datum shift was identical
across the entire quad, you couldn't produce an error that large.

- Ed

Ed McNierney
President and Chief Mapmaker
TopoZone.com / Maps a la carte, Inc.
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
ed at topozone.com
(978) 251-4242

________________________________

From: proj-bounces at lists.maptools.org
[mailto:proj-bounces at lists.maptools.org] On Behalf Of Michael P Finn
Sent: Friday, January 27, 2006 12:52 PM
To: PROJ.4 and general Projections Discussions
Cc: geotiff at lists.maptools.org; proj at lists.maptools.org;
gdal-dev at lists.maptools.org; proj-bounces at lists.maptools.org
Subject: Re: [Proj] Dataset mismatch?

>From my colleague Lynn Usery (USGS/ Geospatial Information Office).
Mike Finn

Using 2 points is not sufficient. The user should use a grid of at least

20 x 20 points (400 total points) over the quad. Transform those between

the datums and resample the pixels based on this approach. This is a
simple operation in Imagine which automatically locates the control
points then applies the datum tranformation, interpolates and resamples
the data. Of course Jarrett said no commercial software, so he must find

a way to implement this process with open source code.

To perform the datum transformation, two points is just not enough to
handle the differences between the two datums. NGS used thousands of
points in the transformation to determine the NADCON shifts between NAD
27 and NAD 83 for the US.

Lynn

"Jarrett L. Redd" <jarrett_l_redd at yahoo.com>
Sent by: proj-bounces at lists.maptools.org

01/27/2006 02:59 AM
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Subject
[Proj] Dataset mismatch?

Howdy...

Please forgive the long cross-posting.  I'm new to this and don't know
exactly
who will have a possible answer for this issue of mine.

I'm a volunteer working on a avalanche terrain and runout mapping
project for
potential use by our mountain search and rescue teams.  I'm using DRG
24k
topographical geotiff slices in UTM NAD27 and then processing 1/3
arc-sec
elevation .adf files in lat/lon NAD83 to match up and plot slopes and
such.
All this is downloaded from the NED seamless website.

Problem is, the two data sets don't match up precisely.  That is, the
features
on the topo seem to match up precisely with elevation data in some
places and
not so precisely in others.  At the worst, the error is around 500 feet.
I'm
using "libproj" to convert coordinates between the two datums.  I'm
currently
processing a section of Colorado, and I'm building "libproj" to include
the
"conus" correction file.  I've also verified my coordinate conversions
are
correct by comparing against openEV and topoUSA.  I'm also using
"libgdal" to
pull out the elevation data and "libgeotiff" to grab the image raster
data and
geo tags.

However, like I said, I'm new to this.  I'm converting the NW and SE
corners of
the geotiff into NAD83 and then interpolating for each pixel to match up
with
the elevation data using the origin and resolution of the various
elevation
pieces.  I have a sneaky feeling that life isn't that simple.  Am I
missing
some fancy projection to correct for curvature of the earth or
something?  Or
is this just an inherent mismatch between the data sets?  Or both?
Other
suggestions to try?

Please don't suggest using a commercially available mapping package
since we
have no money and I need to do very extensive data processing once I
have the
data sets matching up properly.

Thanks.

-Jarrett

P.S.  Here's an example of a nearly 500 foot mismatch error:

A tiny rock spire on the topo map:
13 0394704 4334970

And the corresponding spike in elevation data:
13 0394571 4335039

[Coords provided by OpenEV cursor]

I don't expect a perfect matchup, but the worst areas need to be
corrected
somehow to make the avalanche terrain maps useful.

I can email an example image showing the error if someone is really
interested.

Thanks again.

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